o 



THE PERIODIC GROWTH OF SCALES IN GADID.E 



the scale of a young fish tlie largest corpuscles are much smaller as compared 

 with the largest corpuscles of a scale from an older fish. 



The long axis of corpuscles does not present a uniform direction throughout 

 the extent of the scale. The direction of the long axis generally agrees with 

 the direction of the fibres of the folia to which the corpuscle belongs. It has 

 already been noticed that fibres of consecutive folia of the scale cross one 

 another frequently at right angles, and the same thing has taken place for the 

 corpuscles which belong to these folia. Corpuscles of one, two, or several con. 

 secutive folia frequently become fused together. Corpuscles represent products 

 of a crystalline nature, and exhibit a series of concentric lines which succeed 

 one another from the centre to the periphery. This is not true for all 

 corpuscles, as some have the appearance of vitreous substances, are perfectly 

 homogeneous, and show no trace of concentric lines. 



Baudelot concludes from his observations that the corpuscles are crystalline 

 deposits effected in the tissues of the scales, and more or less modified by this 

 tissue. They are of the same nature as the artificial products, studied firstly 

 by Rainey* and then by Harting.f 



11. The formation and groicth of scales. Scales only appear subsequently 

 to hatching, sometimes a long time after this has taken place ; for example, in 

 young eels measuring 7-8 centimetres in length they have not yet appeared. 

 The scale originates as a spot of dermal calcification, which extends little by 

 little, and thus comes to constitute a small solid lamella, which represents the 

 primitive scale. The first lamella, once formed, sometimes remains closely 

 united to the surrounding tissue, sometimes acquires a certain mobility ; but 

 this mobility is never complete, and the scale always retains intimate connec- 

 tions with the dermis by its internal surface and by its margins, and the 

 external surface itself frequently shows adhesions at the free margin. The 

 young scale grows by the addition of new layers of increasing size, which add 

 themselves successively to its internal face. This mode of growth explains 

 how it is that the scale is considerably thicker towards the centre, and much 

 thinner and less calcified at the periphery. At the internal surface of the 

 scale, and at its margins, tracts of connective tissue are found, l)y means of 

 which the scale adheres to the pouch in which it is contained ; but at the 

 external border, on the other hand, the line of demarcation between scale and 

 dermal pouch becomes more and more marked. As to the subsequent pro- 

 gress of calcification, one can establish that it extends from the exterior 

 towards the interior, and from the periphery of the scale towards the centre. 

 In each layer the calcification is more comj^lete on the border than in the 

 central portion. These calcifications unite with each other, and constitute 

 the calcareous crust of the scale surface. 



As to the concentric ridges and spines, these appear successively on the 

 borders of the scale as that gradually extends itself. One has to admit that 



* "On the mode of formation of tlie shells of animals, of bone, and of several other 

 structures, by a process of molecular coalescence, demonstrable in certain artificially 

 formed products." Rainey, 1858. 



t Harting, "Further Experiments and Observations." Quart. Journal of Microscop. 

 Science, n.s., vol. i. (1861) p. 23. 



